Many countries in which telecommunication networks, and particularly wireless telecommunication networks, operate have call interception or wire-tap laws. From time to time, such laws require the telecommunication service provider to unobtrusively intercept specified calls taking place within a country's jurisdiction. However, in wireless telecommunication networks, and particularly in digital cellular telecommunication networks, true unobtrusive interception is often difficult to achieve or is achieved by wasteful use of the electromagnetic spectrum.
FIG. 1 shows a block diagram which illustrates call interception in a prior art Global System for Mobile communications (GSM) digital cellular radio telecommunication network. In this prior art network, a subscriber unit (SU) 10 engages in digital RF communication with a base transceiver station (BTS) 12 during a call. The BTS 12 is physically located on or near the surface of the earth within radio range of the SU 10. BTS 12 passes the communication through a base station controller (BSC) 14 to a nearby mobile switching center (MSC) 16 where two half-calls are connected together. Accordingly, for the vast majority of calls, both the SU 10 engaged in the call and the MSC 16 which connects two half-calls together reside within the jurisdiction of a single country. Moreover, for a vast majority of calls, call paths to an SU 10 in a country's jurisdiction pass through an MSC 16 which is also in the country's jurisdiction.
Worldwide telephony networks convey a single digital voice communication using the well known DS-0, 64 kbps, PCM standard. This standard protocol allows an accurate reconstruction of voice band analog signals. However, this standard protocol does not meet the needs of wireless communication because the dedication of outgoing and incoming 64 kbps channels to each call wastes the scarce electromagnetic spectrum which must be shared by all who use it. Consequently, digital wireless networks prefer to compress voice communication and transmit the communication at a lower data rate through a narrower bandwidth RF channel to conserve the spectrum.
Lossless compression would be a highly desirable form of data compression because it would permit expansion or decompression to precisely reconstruct an original data stream. However, lossless compression typically achieves only a modest reduction, for example less than 50%, in data quantity, data rate, and channel bandwidth. Accordingly, wireless telecommunication networks typically prefer to use a form of lossy data compression to achieve greater reductions in data quantity, data rate, and channel bandwidth.
Wireless telecommunication networks often employ the use of transcoders or vocoders to analyze human speech at a source node using sophisticated modeling techniques which reflect human speech capabilities. Transcoders compress voice using lossy compression techniques. Well known speech analysis techniques allow digitized speech of acceptable quality to be transported at data rates much lower than 50% of the DS-0 rate. Thus, the prior art network depicted in FIG. 1 uses a 13 kbps data rate for RF links. The use of lower data rates than DS-0 permits the telecommunication network to transport more human speech in the form of calls in a given geographical area using a given amount of spectrum. However, the lossy nature of these compression techniques prevents precise reconstruction of original voice signals.
As illustrated in FIG. 1 for a conventional wireless telecommunication network, a wireless half-call is routed through a high quality (HQ) transcoder 18 for decompression. Decompressed voice communication from two half-calls are connected together in the MSC 16. When the call is to be intercepted, this connection is formed through a conference bridge 20. A third port of the conference bridge 20 provides combined, decompressed voice communication from the two half-calls for routing to a monitoring center, as illustrated at call path 22.
The MSC 16 couples to the wire-based public switched telecommunications network (PSTN). A vast majority of communication with a typical SU 10 is conducted with telephonic devices coupled to the PSTN through a call path 24. Less often SU 10 communicates with an SU 10' supported by the same or other BSCs 14. When a second half-call is associated with SU 10', the second call path is routed through another HQ transcoder 18 for recompression prior to RF transport to SU 10'. The quality of communication suffers when SU 10 communicates with an SU 10' because two compression/decompression cycles are experienced. Since reconstruction of an original signal after a single compression/decompression cycle is not precise, reconstruction of an original signal after two sequential compression/decompression cycles is even less precise. However, since HQ transcoders are employed, only a minor signal quality degradation results from two compression/decompression cycles. Whether or not SU 10 is communicating with another SU 10' or with a telephonic device coupled through the PSTN, the signal quality is the same as it would be whether or not the call is being intercepted. Thus, calls are intercepted unobtrusively.
This conventional wireless communication network undesirably wastes the spectrum due to its reliance upon HQ transcoders. While HQ transcoders permit significant reductions over DS-0 rates, they fall far short of implementing maximal lossy compression and decompression algorithms. Maximal lossy voice compression and decompression algorithms are well known. Such algorithms permit the transmission of acceptable quality voice communication at data rates as low as 2.4 kbps. However, signal reconstruction after two compression/decompression cycles is noticeably inferior to signal reconstruction after one compression/decompression cycle. The prior art telecommunication network avoids using maximal lossy compression, at least in part, because the double compression/decompression cycles would yield unacceptably low quality voice signals.
In addition, the prior art wireless communication network is undesirable because it requires completion of a compression/decompression cycle for each SU half-call prior to connecting half-calls together in an MSC, whether or not the call will be intercepted. This requirement causes the prior art telecommunication network to avoid maximal lossy compression. However, in a network which uses maximal lossy compression to achieve improved spectrum usage, additional compression/decompression cycles are avoided to maintain reconstructed voice signal quality. In such a network call interception could become obtrusive rather than unobtrusive if additional compression/decompression cycles were required to intercept a call.